Digital Cameras use a "sensor" to capture images in much the same way that older cameras used film. When it comes to cleanliness, film cameras had fewer problems because any dust that landed on the film typically moved along with the film as it was advanced through the camera, so only one or two frames might suffer as a result of that dust. Digital camera sensor do not move and therefore do not create the same opportunity to eliminate dust. This creates the potential for a multitude of images being affected by even one spec of dust or junk (hair, lint, etc...) on the sensor. Although many modern cameras include a sensor cleaning system, those systems do not eliminate the need for a periodic cleaning to remove stubborn particulate that the built-in system cannot shake loose.